Abelson virus transforms both fibroblasts and lymphoid cells, and has about 3.5 kbp src specific sequences. By isolating deletion mutants from the cloned Abelson virus DNA, transformation-specific regions are being determined. The Bgl II 1.5 kbp fragment-deleted mutant did not transform fibroblasts. Another mutant, which deletes Bgl II 0.8 kbp fragment near 3' end of the genome, transformed both fibroblasts and lymphoid cells. These data show that 2.7 kbp DNA sequences, except 0.8 kbp Bgl II fragment, are enough for both types of transformation.